The invention relates to fuel cells and, more particularly, to fuel cells arranged in a stack and held in compression.
Fuel cell stacks typically comprise a plurality of fuel cell assemblies stacked one upon the other and held in compression with respect to each other. Typically, each fuel cell assembly comprises an anode layer, a cathode layer, and an electrolyte interposed between the anode layer and the cathode layer. Fuel cell stacks require a significant amount of compressive force to squeeze the cells of the stack together. This force comes about from the internal gas pressure of the reactants plus the need to maintain good electrical contact between the internal components of the cell. Typically, the per area unit force is about 200 psi total. This force must be distributed evenly over the entire active area of the cell (typically 500-1,000 square centimeters for automotive size stacks). Thus, the total compressive force of these size stacks is between 16,000 and 32,000 pounds. The challenge for the designer of the stack compression mechanism is to distribute the compressive force uniformly over the cell active area.
Prior art attempts to provide this uniform compression distribution have included rigid end plates with external tie rods, rigid end plates with band clamps, semi-rigid end plates with a cavity for a gas bladder, and rigid end plates with internal tie rods passing through the cells.
In the rigid end plate with external tie rods, threaded tie rods extend from the perimeter of an upper end plate along the outside of the stack to the perimeter of the lower end plate so that the total compressive force is carried by the tie rods. The end plate must be thick enough so that a small (about less than 1 mil per cell) total deflection is achieved. The disadvantage of this system is that the end plate must be very thick as compared with all other options since the total end plate span is the largest and no other method is employed to generate even force over the entire plate area.
In the rigid end plate with band clamps, one or more band straps are wrapped around the end plates to provide a degree of support at the center of the upper end plate. This arrangement has the advantage that the bands are thin straps of metal resulting in a lesser volume consumed at the stack exterior as compared with external tie rods but has the disadvantage that it is difficult to realize a significant support at the center of the upper end plate with the straps.
In the semi-rigid end plate with a cavity for a gas bladder, the lower face of the upper end plate is hollowed out, a bladder is positioned in the end plate cavity, and the bladder is pressurized to provide the desired compressive loading of the stack. The upper end plate itself is now allowed to bend somewhat while the bladder maintains uniform force distribution over the total plate area. This arrangement has the advantage that the structural component of the upper end plate can be made thinner since it is allowed to flex considerably but has the disadvantage that it requires a cavity in the end plate with the result that the overall thickness of the end plate is significantly increased.
In the rigid end plate with internal tie rods through the cells, the tie rods extend through the center of the cells to allow the placement of the tie rods nearer to the center of the end plate. Now the total span of the bending force is not extended over the entire width of the upper end plate but rather a shorter span is achieved. This arrangement has the advantage of reducing span length of the upper end plate resulting in the ability to use a thinner end plate but has the disadvantage that it requires complex bipolar plate sealing mechanisms to enable the tie rods to pass through the cells.
This invention is directed to the provision of an improved fuel cell assembly.
More specifically, this invention is directed to the provision of an improved compression method and apparatus for a fuel cell stack.
The invention is directed to a fuel cell of the type comprising an upper end plate assembly; a lower end plate assembly; at least one electrochemical fuel cell assembly interposed between the upper and lower end plate assemblies and including an anode layer, a cathode layer, and an electrolyte interposed between the anode and cathode layers; and clamp means operative to compressively clamp the stack.
According to the invention apparatus, one of the end plate assemblies comprises an end plate confronting the fuel cell assembly and pressed against the fuel cell assembly by the clamp means; a plurality of discrete force exerting devices positioned in the end plate at spaced locations in the end plate and selectively movable relative to the end plate in an axial direction generally transverse to the general plane of the end plate; and holding means operative to maintain each force exerting device in any axial position to which it is moved relative to the end plate. With this arrangement, the discrete force exerting devices may be selectively moved relative to the end plate to assure a substantially uniform compressive loading across the area of the fuel cell assembly and the end plate itself may be relatively thin and allowed to flex.
According to a further feature of the invention apparatus, the force exerting devices and the holding means comprise screws threaded through bores in the end plate and bearing at free ends thereof against a confronting face of the fuel cell assembly. With this arrangement the screws may be selectively tightened to provide a substantially uniform torque in each screw whereby to assure a uniform compressive loading across the area of the fuel cell assembly while allowing the use of a relatively thin end plate which is allowed to flex.
According to a further feature of the invention apparatus, the one end plate assembly comprises the upper end plate assembly and the end plate comprises an upper end plate. The principles of the invention may be applied to either the upper or the lower end plate assembly or both end plates. The principles of the invention are most readily and advantageously applied to the upper end plate assembly.
According to a further feature of the invention apparatus, the upper end plate assembly further includes a distributor plate overlying the fuel cell assembly and interposed between the fuel cell assembly and the upper end plate and the lower ends of the screws bear against the upper face of the distributor plate. With this arrangement selective tightening of the screws assures a substantially uniform compressive loading across the area of the distributor plate and thereby across the area of the fuel cell assembly.
According to a further feature of the invention apparatus, the clamping means comprises tie rod assemblies interconnecting the upper and lower end plate assemblies. The compression arrangement of the invention allows conventional external tie rod assemblies to be utilized in combination with a relatively thin upper end plate.
The invention methodology relates to a method of reducing the thickness and thereby the weight of one of the end plates of an electrochemical fuel cell stack of the type comprising an upper end plate; a lower end plate; at least one electrochemical fuel cell assembly interposed between the upper and lower end plates and including an anode layer, a cathode layer, and an electrolyte interposed between the anode and cathode layers; and clamp means operative to compressively clamp the stack.
According to the invention methodology, a plurality of discrete force exerting devices are positioned in the one end plate at spaced locations in the one end plate with each force exerting device being selectively movable relative to the one end plate in an axial direction generally transverse to the general plane of the one end plate; holding means are provided which are operative to maintain each force exerting device in any axial position to which it is moved relative to the one end plate; and each force exerting device is selectively moved relative to the one end plate to assure substantial uniform compressive loading across the area of the fuel cell assembly. This methodology allows the use of a relatively thin and relatively lightweight end plate which may flex in response to the selective movement of the force exerting devices while the force exerting devices act to provide the uniform compressive loading across the area of stack.
According to a further feature of the invention methodology, the force exerting devices and the holding means comprise screws threaded through threaded through bores in the one end plate and bearing at free ends thereof against a confronting face of the fuel cell assembly and the selectively moving step comprises selectively tightening the screws to provide a substantially uniform torque in each screw. This methodology assures a uniform compressive loading across the area of the fuel cell assembly while allowing the relatively thin and relatively lightweight end plate to flex.
According to a further feature of the invention methodology, the one end plate comprises the upper end plate, the fuel cell stack further includes a distributor plate overlying the fuel cell assembly and interposed between the fuel cell assembly and the upper end plate, and the lower ends of the screws bear against the upper face of the distributor plate. With this methodology, selective tightening of the screws assures a substantially uniform compressive loading across the area of the distributor plate and thereby across the area of the fuel cell assembly.